WASHINGTON (Reuters) – Whenever Republican U.S. presidential candidate Mitt Romney has been down and in need of an enforcer, one has been there.

Restore Our Future, the independent â€œSuper PACâ€ that supports Romneyâ€™s campaign, has been a machine of destruction in the race for the Republican presidential nomination, swamping Republican rival Newt Gingrich with attack ads each time he has seemed to threaten Romneyâ€™s frontrunner status.

Until late on Tuesday, the engines behind the PAC – its donors – were largely secret.

But the groupâ€™s financial reports to the Federal Election Commission (FEC) reveal that private equity executives, hedge fund managers and other financial heavyweights are key contributors to the fundraising juggernaut that Restore Our Future has become.

Political action committees (PACs) are groups with great clout in U.S. politics that are legally separate from candidates. In this campaign, they have unleashed negative advertising against the rival of the candidate they favor.

The filings reveal that Restore Our Future had $23.6 million in the bank as of December 31 – more than the Romney campaign itself. The PAC has spent $17.4 million in support of Romney, including nearly $11 million in Florida before Romneyâ€™s decisive victory in the Republican primary there on Tuesday.

The groupâ€™s filings reveal a broad base of donors who have given $100,000 or more, taking advantage of the lack of limits on donations to such groups. Campaigns are limited to $2,500 donations per donor.

Among the most generous contributors to Romneyâ€™s cause have been people who share Romneyâ€™s investment background. Romney, a former Massachusetts governor, co-founded Bain Capital, a private equity firm, and has emphasized his business experience in touting his ability to create jobs and improve the economy.

Three hedge fund managers pitched in $1 million to the pro-Romney PAC: Tiger Managementâ€™s Julian Robertson, Elliot Managementâ€™s Paul Singer and Renaissance Technologyâ€™s Robert Mercer. Miguel Fernandez, chairman of a healthcare private equity firm, gave $500,000. MBF Family Investments, which shares an address with Fernandezâ€™s investment company, gave another $500,000.

The list of major donors to Restore Our Future is studded with other veterans of Wall Street and the investment community.

Romney, mocked by Gingrich as a â€œMassachusetts moderate,â€ has received the support of conservative money men such as Bob Perry, a Texas home builder, and William Koch, a billionaire investor whose family is active in conservative politics.

â€œYou would think they would be backing much more conservative candidates,â€ said Fred Zeidman, a Houston fundraiser for Romney. â€œIt seems to me what they are saying, â€˜Hey weâ€™ve got to beat Barack Obama, and this is the way we are going to do it.â€™â€

Fourteen individuals contributed at least $250,000 to Restore Our Future. That is far more than the number of such donors who have given to competing Super PACs.

Winning Our Future, the group allied with Gingrich, has depended largely on one couple: casino billionaire Sheldon Adelson and his wife, Miriam, who each chipped in $5 million in January, a month after their two daughters had given a total of $750,000.

Our Destiny, a Super PAC that backed former Utah governor Jon Huntsman when he was in the race, raised $2.7 million in the fourth quarter of 2011 – $1.9 million of it from Huntsmanâ€™s father, industrialist Jon Huntsman Sr.

Wyoming billionaire Foster Friess contributed nearly half the $729,935 raised by the Red White and Blue Fund, which supports another Republican, former Pennsylvania U.S. senator Rick Santorum. The financial gap between Romneyâ€™s PAC and the rest of the field is likely to widen, said campaign finance analyst Anthony Corrado.

â€œI expect it would be much easier for the Romney Super PAC to go out and solicit big contributions after a win in Florida, and the other Super PACs will be even more dependent on the largesse of a few wealthy individuals to continue to be willing to write large checks,â€ said Corrado, a government professor at Colby College in Maine.

Restore Our Future is based in Washington and is run from the law firm Clark Hill by treasurer Charles Spies. It has no offices in any of the 50 states and no staff presence on the campaign trail. Rivals describe it as a monolithic campaigning machine, with tight-lipped members, lots of money and a favorite target: Gingrich.

â€œThatâ€™s a compliment as long as itâ€™s an effective machine,â€ said Spies, former counsel for Romneyâ€™s 2008 campaign.

The group has outspent its rival PACs by far, dumping millions of dollars into TV, Internet, direct-mail and other types of advertisements – nearly all of them attacking Gingrich.

The ads casting Gingrich as an erratic Washington insider with questionable ethics helped send Gingrichâ€™s campaign into a dive just before the Iowa caucuses, the first contest in the race to pick a Republican nominee to face Democratic President Barack Obama in the November 6 election.

Restore Our Futureâ€™s anti-Gingrich ads appear to have had a similar effect in advance of Tuesdayâ€™s vote in Florida, where the PAC had spent nearly $11 million.